1. Field of the Invention
The present invention relates to a semiconductor fabrication apparatus such as a chemical vapor deposition (CVD) apparatus, and more particularly, to an improvement in a handler for loading and unloading wafers into and from a process chamber.
2. Description of the Related Art
In a semiconductor fabrication apparatus having a process chamber, a handler is provided to load wafers into the process chamber or to unload wafers from the process chamber.
In one such semiconductor fabrication process, for example, a thin film growth technique, generally referred to as a chemical vapor deposition (CVD) method, forms a thin film of polycrystalline, such as silicon (Si), silicon nitride (Si.sub.3 N.sub.4) and silicon dioxide (SiO.sub.2), on a semiconductor wafer using a chemical reaction in a high temperature environment or an activated environment. The chamber of the CVD equipment, which forms a thin film on a wafer using this CVD method, must be kept in a vacuum state and is equipped with a load fork which loads wafers into the chamber or unloads them therefrom.
The CVD equipment generally consists of two chambers, that is, a load chamber and a process chamber. The load chamber is provided to pre-locate wafers to be processed or temporarily house or store wafers that have been processed, prior to their transfer to another process stage. The process chamber forms a thin film on a semiconductor substrate using an evaporation process. The load chamber includes an aligning plate on which the wafers to be processed are located, and a load fork which picks a wafer up from the aligning plate and loads it into the process chamber. The load fork also unloads the wafer from the process chamber that has been processed and lays it down on the aligning plate.
When the aligning plate is horizontal during the time the wafer is in the process chamber, the aligning plate is said to be in an `up` state. When the load fork places and aligns a wafer on the aligning plate prior to loading the wafer in the process chamber, or aligns the wafer unloaded from the process chamber on the aligning plate, the aligning plate is vertical, and it is said to be in a `down` state. The position of the aligning plate, whether seated horizontally or erected vertically, is detected by at least two micro limit switches and the result of the detection is provided to an equipment controller.
As for the load fork, when the load fork is dropped or is in stand-by, it is said to be in a `down` state. The dropped or stand-by state of the load fork occurs when the wafers are placed or loaded in the load chamber, or when the wafers are placed on the wafer chuck, or when the wafers are taken from the process chamber. When the load fork picks up the wafers after processing and transfers them, the load fork is said to be in an `up` state. The change in state of the load fork may be also detected by the two micro limit switches and the result of the detection is transferred to the equipment controller.
FIG. 1 shows an assembly for driving the load fork which constitutes one of the handlers of a CVD equipment. Referring to FIG. 1, there is shown actuator anchors 1, crank linkages 2, speed adjustors 3, air connection parts 4, eccentric wheel washers 5, micro limit switches 6, and sensing bars 7 for the micro limit switches, respectively.
In the CVD equipment, the micro limit switches 6 detect whether the load fork or the aligning plate is `down` or `up`. When the driving shaft of the load fork/aligning plate pushes down the sensing bars 7 of micro limit switches 6, then electrical signals are supplied to a data input board. The position of the load fork and the aligning plate is thus displayed on a diagnostic display. The electrical signals generated from the micro limit switches 6 are also supplied to the equipment controller. The controller then detects the signals and carries out a handling sequence control.
As described above, if the sensing bars 7 for the micro limit switches 6 are pushed down and electrical signals of a predetermined level are generated from normal open terminals of the micro limit switches 6, the equipment controller can determine whether the load fork or the aligning plate is in the `up` or `down` state.
As this equipment is operated continually in the production line, however, the sensing bars 7 frequently break down since they are formed with a thin plate, or the contact points of the switches 6 frequently burn out. The equipment controller, therefore, can not detect the position of the load fork or the aligning plate. A problem arises in that it becomes impossible to carry out an accurate sequencing control.
More specifically, when the load fork is actually in the `down` state, if the sensing bar 7 for the micro limit switch 6 is broken, or the contact points of the micro limit switch 6 are burned out, the controller can not detect if the load fork is actually `down`, so that an error occurs in the sequence control for the load fork.
When an error occurs in the sequence control of the load fork or the aligning plate, the error may be detected by using a DVM (digital voltage meter) to determine whether the error is due to the breakage of the micro limit switches 6. Such an error detection procedure takes time, however, resulting in increased maintenance time for the equipment.